Mechanical engineers plan and design tools, engines, machines, and other mechanical systems that produce, transmit, or use power. They may work in design, instrumentation, testing, robotics, transportation, or bioengineering, among other areas. The broadest of all engineering disciplines, mechanical engineering extends across many interdependent specialties. Mechanical engineers may work in production operations, maintenance, or technical sales, and many are administrators or managers. Approximately 277,500 mechanical engineers are employed in the United States.